1. Field of Invention
This invention relates to an image data processing method for use with a nuclear magnetic resonance (hereinafter called "NMR") tomograph for observing the internal structure of an object as a sectional image by using the NMR phenomenon; and more particularly, to an offset correction of the image data of the tomogram.
2. Description of Prior Art
The data obtained from an NMR tomograph is not a practical useable image as it is because the data contains various noises. Most of these noises can be corrected by a numerical processing using a variety of physical properties, but either noises intrinsic to the tomograph or transient noises usually remain uncorrected by the numerical processing.
Also, the DC level of the image data will vary for each view, due to fluctuations in the gain of an amplifier and the offset value of an analog-digital converter. This variation forms a salience of the DC component in the image and establishes a bright spot or line, as an arch fact, to seriously degrade the quality of the image.
In order to solve this problem, there has been widely used a method wherein such data, as can be interpreted to contain few image signals, are sampled either in succession to or prior to the image data so that their average value is used as an offset value of the view.
However, this method is accompanied by a problem, wherein the offset cannot be sufficiently corrected in case it has a gradient. In case the gradient of the offset is dependent upon RF pulses, gradient magnetic field fluctuations, etc, more specifically, it is common among the views that a sufficiently effective correction can be accomplished by averaging the signal-less views and portions and by data processing the averaged one as an offset file. However, even this method is still defective in that an uncorrectable offset remains in case the offset has a gradient which is irregular for each view and exhibits gentle and slight changes in the view.